Various control methods are conventional for maintaining the oxygen content of the exhaust-gas mixture as precisely as possible at a value λ=1, in order to achieve an optimum catalytic three-way conversion.
The controller approaches differ depending upon the number of catalytic converters used, as well as the quantity and type of λ-sensors employed. Conventional is a front control loop having a two-step control as a function of the signal from a voltage-jump λ-sensor, or having a continuous control as a function of the signal from a broadband λ-sensor upstream of the catalytic converter. For example, in order to correct deviations of the front sensor, often a second control loop (rear control loop) is implemented, which is based on the signals of a λ-sensor downstream of the catalytic converter. Usually a PI or PID controller is involved.
A problematic disadvantage of these approaches is believed to be that the rear controller first reacts to the mixture with a corrective intervention when the sensor signal exhibits a deviation from the predetermined setpoint value.
However, such a deviation also means that the catalytic converter is not or has not operated in its optimum range, and therefore as a rule is associated with unwanted emissions.
That is why it has already been proposed to record or to model the performance of the catalytic converter to the effect that a corrective intervention is already carried out even before a deviation is detected at the rear sensor. In particular, such an approach usually includes a catalytic-converter model for recording the instantaneously stored oxygen quantity (oxygen model), since the oxygen stored in the catalytic converter substantially influences the conversion and subsequently the reaction of the sensor signal.
Moreover, different methods have been proposed to use information about the instantaneous oxygen state of the catalytic converter for improving the λ-closed-loop control, e.g., by modification or supplementation of the control algorithms. The problem in these approaches is that a high accuracy of the ascertained instantaneous oxygen state of the catalytic converter is necessary, and the correlation between this oxygen state and the conversion efficiency of the catalytic converter must be described precisely, since otherwise, due to deviations between the real and the modeled performance, the reaction of the controller may be inadequate, therefore leading to unwanted emission of pollutants.
German Published Patent Application No. 103 39 063 describes a method for mixture control in which a breakthrough of pollutants in a catalytic converter is largely avoided by ascertaining the degree to which the catalytic converter is loaded with oxygen, and upon reaching limiting values for the loading, in each case to switch over between rich and lean mixture. A model of the oxygen-storage capacity of the catalytic converter is used, which calculates a value for the oxygen loading of the catalytic converter as a function of input-λ-values and catalytic-converter parameter values. A change in the mixture is initiated as a function of the calculated value of the oxygen loading and a rich and/or lean breakthrough at the catalytic converter ascertained by the oxygen sensor.